PURPOSE: To easily measure the viscosity of a fluid even if the fluid is corrosive to eliminate the need for maintenance control such as disassembling, cleaning, etc., by detecting the phase of a diaphragm provided to a pipe in contact with flowing fluid.

CONSTITUTION: The viscous fluid flows in a main pipe 1 in contact with the diaphragm 24, which is vibrated by a vibration oscillator 26 at a constant vibration frequency to a constant amplitude. This vibrator 24 contacts the viscous fluid, so the diaphragm vibrates at a different frequency from the vibrations from the vibration oscillator 26. For the purpose, a detection end 28 detects the frequency of the vibrations of the diaphragm 24 and a phase difference detector 32 detects the phase difference between the phase detected by a phase detector 30 and that of the vibrations that the vibration oscillator 26 applies to the diaphragm 24. A signal of the phase difference is inputted to an internal storage device 34, a controller 38 performs control according to an indication from a control board 36, and an arithmetic unit 40 performs arithmetic operation by using the known data stored in the internal storage device 34 to output the numeral of the viscosity.